1. Field of the Invention
The present invention relates to an image processing apparatus which creates composite image information by embedding, in visible main image information (e.g., a human facial image), another additional sub-information (e.g., security information) in an invisible state, and restores the embedded sub-information from the recorded composite image information, and a printed material on which the composite image information created by the image processing apparatus is printed.
2. Description of the Related Art
Recently, with the trend toward computerization of information and the proliferation of the Internet, increasing importance has been attached to an electronic watermarking technique, digital signature technique, and the like to prevent counterfeiting and alteration of images. The above electronic watermarking technique is a technique of embedding additional sub-information (sub-image information) in main image information in an invisible state. For example, the electronic watermarking technique is used for a personal authentication medium such as an ID card on which personal information is recorded or a literary work. The electronic watermarking technique can prevent unauthorized copying, counterfeiting, and tampering of a personal authentication medium and literary work, thereby protecting the personal information on the personal authentication medium and the copyright of the literary work.
For example, Jpn. Pat. Appln. KOKAI Publication No. 9-248935 discloses en electronic watermarking technique of embedding data in image data output onto a printed material by using the characteristics of high spatial frequency components and color difference components which are difficult for man to perceive. Jpn. Pat. Appln. KOKAI Publication No. 2001-268346 discloses a printing apparatus for electronic watermarks that can be recognized through optical filters.
Recording apparatuses for recording composite image information created by the above electronic watermarking technique on a medium include, for example, a recording apparatus based on a sublimation/thermal transfer recording scheme and a recording apparatus based on a fusion thermal transfer recording scheme.
In general, in the sublimation/thermal transfer recording scheme, materials that can be dyed with sublimable materials are limited. For this reason, the sublimation/thermal transfer recording scheme can be adapted to only limited recording media; the degree of freedom of choice regarding recording media on which images are recorded is low. According to the sublimation/thermal transfer recording scheme, therefore, materials that can be used as recording media are limited. This tends to degrade security. In addition, sublimable dyes generally have poor image durability, e.g., poor light resistance and poor solvent resistance.
In contrast to this, in the fusion thermal transfer recording scheme, a material having good light resistance can be generally selected as a coloring material. The fusion thermal transfer recording scheme therefore allows a high degree of freedom of choice regarding recording media. In the fusion thermal transfer recording scheme, therefore, a high-specialty recording medium can be used. This makes it possible to improve security. The fusion thermal transfer recording scheme, however, uses a dot area gradation method of performing gradation recording by changing the sizes of transferred dots. With this scheme, therefore, it is difficult to realize as high gradation performance as that with the sublimation/thermal transfer recording scheme.
In order to solve this problem, for example, Jpn. Pat. Appln. KOKOKU Publication No. 6-59739 discloses a method of recording transferred dots in a so-called staggered array (this method will be referred to as an alternate driving/recording scheme hereinafter).
As described above, the electronic watermarking technique is applied not only to electronic information but also to printed materials such as driver's licenses, credit cards, and membership cards. For example, Jpn. Pat. Appln. KOKAI Publication No. 2001-274971 discloses an electronic watermarking technique of printing an image upon embedding a given image therein. According to this technique, the embedded image is difficult for man to perceive, and can be restored even after the image is printed.
A technique is also disclosed in Japanese Patent No. 2840825, in which a hologram film is formed on a personal authentication medium such as a driver's license, credit card, or membership card to prevent copying thereof.
In another known technique, security is improved by, for example, printing a facial image (to be referred to as a ghost image hereinafter) lower in density than a normal facial image on a side of the normal facial image.